Steam cracking processes are used to produce ethylene and propylene as basic petrochemicals. Steam cracking processes also produce olefinic C4 hydrocarbons such as butene-1, butene-2, isobutene, butynes and butadiene. The demand for these compounds as petrochemical feedstocks is less than the quantities produced, and hence various approaches have been suggested for converting surplus C4 olefins into more valuable products.
One process for conversion of olefinic C4 streams is to carry out metathesis with ethylene to form propylene. This is a particularly attractive process, as it gives the steam cracker operator the flexibility to adjust the ratio of ethylene to propylene that is produced in the complex. The preferred feed for a metathesis plant is a stream rich in butene-2. Isobutene, butynes and butadiene are not desirable feeds to the metathesis process as these compounds cause deactivation of the metathesis catalyst and/or higher coke make in the metathesis process. Butene-1 is not a desirable feed to the metathesis process as it will react with butene-2 to produce pentenes and with itself to produce hexenes. These compounds are low-value byproducts. Some form of pretreatment is, therefore, normally applied to the olefinic C4 stream to produce a stream rich in butene-2 that can be used as a metathesis feed.
The current state-of-the-art process for removal of isobutene from a metathesis feed is to react the isobutene to form methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE) or tert-butyl alcohol (TBA). These processes can be carried out at greater than 99.9% conversion of isobutene with negligible conversion of normal butenes and, hence, permits high propylene yields in the metathesis process. The phase out of MTBE as a gasoline additive has reduced the economic advantage of this conversion route and led to a need for new processes that convert isobutene into more valuable products.
An alternative process for the removal of isobutene from a metathesis plant feed is to react away the isobutene in a polymerization process. If the entire C4 stream is subjected to polymerization conditions then substantial conversion of normal butenes also occurs, which reduces the overall yield of propylene. Furthermore, separation of isobutene from normal butenes is difficult as isobutene boils very close to butene-1.
As described below, various methods have been proposed for preparing a metathesis feed from an olefinic C4 stream. There remains, however, a need for improving the value of the products produced from such processes and for reducing the capital and operating costs of such processes.